Microcavity platform for widely tunable optical double resonance

Tunable open-access Fabry–Perot microcavities are versatile and widely applied in different areas of photonics research. The open geometry of such cavities enables the flexible integration of thin dielectric membranes. Efficient coupling of solid-state emitters in various material systems has been demonstrated based on the combination of high quality factors and small mode volumes with a large-range in situ tunability of the optical resonance frequency. Here, we demonstrate that by incorporating a diamond micromembrane with a small thickness gradient, both the absolute frequency and the frequency difference between two resonator modes can be controlled precisely. Our platform allows both the mirror separation and, by lateral displacement, the diamond thickness to be tuned. These two independent tuning parameters enable the double-resonance enhancement of nonlinear optical processes with the capability of tuning the pump laser over a wide frequency range. As a proof of concept, we demonstrate a >THz continuous tuning range of doubly resonant Raman scattering in diamond, a range limited only by the reflective stopband of the mirrors. Based on the experimentally determined quality factors exceeding 300,000, our theoretical analysis suggests that, with realistic improvements, a ∼mW threshold for establishing Raman lasing is within reach. Our findings pave the way to the creation of a universal, low-power frequency shifter. The concept can be applied to enhance other nonlinear processes such as second harmonic generation or optical parametric oscillation across different material platforms.